[PATCH v2 1/2] landlock: Serialize TSYNC thread restriction

[Günther Noack]

On Wed, Feb 25, 2026 at 10:47:33AM +0800, Yihan Ding wrote:
> syzbot found a deadlock in landlock_restrict_sibling_threads().
> When multiple threads concurrently call landlock_restrict_self() with
> sibling thread restriction enabled, they can deadlock by mutually
> queueing task_works on each other and then blocking in kernel space
> (waiting for the other to finish).
> 
> Fix this by serializing the TSYNC operations within the same process
> using the exec_update_lock. This prevents concurrent invocations
> from deadlocking. We use down_write_killable() to ensure the thread
> remains responsive to fatal signals while waiting for the lock.
> 
> Fixes: 42fc7e6543f6 ("landlock: Multithreading support for landlock_restrict_self()")
> Reported-by: syzbot+7ea2f5e9dfd468201817 at syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=7ea2f5e9dfd468201817
> Suggested-by: Günther Noack <gnoack3000 at gmail.com>
> Signed-off-by: Yihan Ding <dingyihan at uniontech.com>
> ---
>  security/landlock/tsync.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/security/landlock/tsync.c b/security/landlock/tsync.c
> index de01aa899751..420fcfc2fe9a 100644
> --- a/security/landlock/tsync.c
> +++ b/security/landlock/tsync.c
> @@ -447,6 +447,13 @@ int landlock_restrict_sibling_threads(const struct cred *old_cred,
>  	shared_ctx.new_cred = new_cred;
>  	shared_ctx.set_no_new_privs = task_no_new_privs(current);
>  
> +	/*
> +	 * Serialize concurrent TSYNC operations to prevent deadlocks
> +	 * when multiple threads call landlock_restrict_self() simultaneously.
> +	 */
> +	if (down_write_killable(&current->signal->exec_update_lock))
> +		return -EINTR;
> +
>  	/*
>  	 * We schedule a pseudo-signal task_work for each of the calling task's
>  	 * sibling threads.  In the task work, each thread:
> @@ -556,6 +563,7 @@ int landlock_restrict_sibling_threads(const struct cred *old_cred,
>  		wait_for_completion(&shared_ctx.all_finished);
>  
>  	tsync_works_release(&works);
> +	up_write(&current->signal->exec_update_lock);
>  
>  	return atomic_read(&shared_ctx.preparation_error);
>  }
> -- 
> 2.51.0
> 

Thank you!

Reviewed-by: Günther Noack <gnoack at google.com>